PATTERNS OF BENTHIC FAUNA AND BENTHIC RESPIRATION ON THE CELTIC CONTINENTAL-MARGIN IN RELATION TO THE DISTRIBUTION OF PHYTODETRITUS

In-situ and shipboard measurements of sediment community oxygen consum ption (SCOC), in conjunction with a study of the distributions of macr o and megafauna and phytopigments, were used to determine and, where p ossible, explain the distribution of labile particulate organic matter (POM) on the NE Atlantic continental slope (Goban Spur, SW Ireland). A specific issue concerned the existence of depocentres of labile POM on the slope caused by lateral transport, a phenomenon that has been f ound previously in the NW Atlantic. The SCOC data from October 1993 an d May 1994 showed a steady decrease with increasing water depth. SCOC values ranged from 5.4 mmol m(-2) at the shelf-break to 0.3 mmol m(-2) d(-1) at 4,500 m depth. No evidence was found for seasonal variation in SCOC. A clear seasonal signal was observed with regard to sediment phytopigments and phytopigment fluxes into sediment traps attached to the benthic lander. The upper-and mid-slope values of both parameters were much higher in May 1994 than in October 1993 and August 1995. Thi s is consistent with the normal spring bloom pattern; but because of t he degraded state of the May phytodetritus in the near-bottom water, r eflected in the lack of a response in SCOC and the low chlorophyll-a c oncentrations, it was concluded that the material was not derived from the overlying photic zone, but instead transported from elsewhere in the benthic nepheloid layer (BNL). In August 1995, the lower slope (>3 ,000 m) had received a strong and fresh phytodetritus pulse (3 g C m(- 2)) forming a mucous layer on top of the sediment. Using phytopigments and sterols as molecular markers, it was shown that the pulse was der ived from an offshore bloom with an important contribution by dinoflag ellates. By contrast, no mucous layer was found on the upper slope sta tions in August 1995. Macrofauna biomass showed a distinct decline fro m the upper slope down to the lower slope conforming to the diminishin g supply of labile POM. The total wet biomass of megafauna reached rel atively high values at the lower slope (>3,500 m) owing to large motil e sea cucumbers. The presence of these ''vacuum-cleaner'' sea cucumber s is considered indicative of the occurrence of phytodetritus pulses. In spite of their assumed adaptation to periodic pulses, the estimated contribution by the sea cucumbers to the total benthic mineralization is minor. When combining data from different years/seasons we observe d decoupling between the food supply to the lower slope and the upper and mid slopes. The major pulse to the former comes from an offshore s ummer bloom. The upper and mid slope appear to be fuelled by spring bl oom material which is subsequently redistributed on the upper slope in a BNL. The quality of the seston in the BNL diminished in the offshor e direction as indicated by the phytopigment concentrations.